The present invention relates to an aqueous fluoroelastomer curable composition and a coated article. In particular, the present invention relates to an aqueous fluoroelastomer curable composition comprising a fluorine-containing copolymer and a specific polyol curing agent, and an article having a coating film formed from such a composition.
A method for curing an aqueous fluoroelastomer coating composition with a polyamine curing agent is known (see JP-B-58-53671 corresponding to U.S. Pat. No. 4,339,553). In general, the coating film formed by this method has good mechanical properties, but low sealing properties. In addition, when such a coating film is used to coat the surface of a roll used in office automation (OA) equipment (e.g. copying machines, printers, etc.), it has drawbacks such as insufficient elasticity, heat resistance, and so on. When the polyamine curing agent is added to a fluoroelastomer curable composition, for example, a coating composition, its pot life is shortened because of gelling, etc. Accordingly, two or more fluid type compositions are practically supplied, but no practically usable one-fluid type composition is available.
When a fluoroelastomer is cured with a polyol curing agent, it is possible to prepare one-fluid type fluoroelastomer coating compositions containing organic solvents (see PCT/JP97/02853). However, in these years, the VOC regulation and the like become severer and severer, and thus environment friendly aqueous coating compositions are desired. However, one-fluid type aqueous fluoroelastomer coating compositions in the form of an aqueous dispersion containing no organic solvent are not known.
One object of the present invention is to provide a one-fluid type aqueous fluoroelastomer curable composition, which can be formulated as an aqueous dispersion, form a cured film having high strength at a relatively low temperature in a short time, and has a long pot life.
Another object of the present invention is to provide a coated article, which does not suffer from the drawbacks of coated articles having a coating film formed from a conventional aqueous fluoroelastomer coating composition, in particular, rolls for OA equipment.
To achieve these and other objects, the present invention provides an aqueous fluoroelastomer curable composition comprising an aqueous fluoroelastomer dispersion and a basic polyol curing agent, and a coated article at least a part of the surface of which is coated with a coating layer formed from such a composition.
Now, the components contained in the composition of the Present invention will be explained.
(A) Aqueous Dispersion of Fluoroelastomer
The aqueous dispersion of a fluoroelastomer is prepared by dispersing a fluorine-containing elastic copolymer in water in a concentration of 10 to 75 wt. % in the presence of a surfactant such as polyoxyethylene, an alkyl phenyl ether, an alkylsulfonate salt, etc.
The fluorine-containing elastic copolymer is a fluorine-containing copolymer comprising repeating units represented by the formula: xe2x80x94CH2xe2x80x94 in the backbone. Typical examples of such a copolymer include fluorine-containing elastic copolymers, comprising vinylidene fluoride, and specific examples of such copolymers are those comprising the following repeating units in the backbone:
at least one repeating unit selected from the group consisting of xe2x80x94CF2xe2x80x94CH2xe2x80x94, xe2x80x94CH2xe2x80x94CH2xe2x80x94 and xe2x80x94CH2xe2x80x94CH(CH3)xe2x80x94 and
at least one repeating unit selected from the group consisting of xe2x80x94CF2xe2x80x94CF(CF3)xe2x80x94, xe2x80x94CF2xe2x80x94CF2xe2x80x94 and xe2x80x94CF2xe2x80x94(ORf)CFxe2x80x94 in which Rf is a fluoroalkyl group having 1 to 6 carbon atoms.
Specific examples of such copolymers include vinylidene fluoride-hexafluoropropylene copolymers, vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymers, ethylene-hexafluoropropylene copolymers, tetrafluoroethylene-propylene copolymers, etc. Such fluorine-containing elastic copolyers are commercially sold under the trade names of xe2x80x9cDAIEL(copyright)xe2x80x9d (by Daikin Industries Ltd.), xe2x80x9cBAITON FLOAM(copyright)xe2x80x9d (by E. I DuPont), xe2x80x9cAFLAS(copyright)xe2x80x9d (by ASAHI GLASS Co., Ltd.), and so on. Among them, the vinylidene fluoride base copolymers are preferable in view of the crosslinkability.
(B) Basic Polyol Curing Agent
Herein, the basic polyol curing agent means a compound or a polymeric compound having at least two hydroxyl groups, in particular, phenolic hydroxyl groups in the molecule, and the curing ability.
Examples of the polyol curing agent includes salts of a basic compound with a phenol compound of the formula: 
or a phenolic resin of the formula: 
wherein Z is xe2x80x94CH2xe2x80x94 or xe2x80x94CH2OCH2xe2x80x94, Y is a hydrogen atom, a halogen atom, or a group of the formula: xe2x80x94R, xe2x80x94CH2OR or xe2x80x94OR in which R is an alkyl group having 1 to 4 carbon atoms, and n is an integer of 0 to 100.
Examples of the basic compound include ammonium, phosphonium, alkali metals (e.g. lithium, sodium, potassium, etc.), alkaline earth metals (e.g. beryllium, magnesium, calcium, barium, etc.), and so on.
Among them, the salts of hydroquinone, bisphenol A, bisphenol AF, resol type phenolic resins, etc. are preferable in view of the properties of the coating films.
As a curing agent, a combination of the above basic polyol and other polyol may be used.
(C) Curing Accelerator
The composition of the present invention may contain the following compound as an optional curing accelerator:
Quaternary Ammonium Salt
A quaternary ammonium salt of the formula:
NR4X or R3Nxe2x80x94Rxe2x80x2xe2x80x94NR3.2X
wherein X is an acid residue or a hydroxyl group, R groups are the same or different and each an alkyl group having 1 to 20 carbon atoms, an halogenated alkyl group having 1 to 20 carbon atoms or an aryl group having 6 to 20 carbon atoms, or two or more R groups together form a carbon ring or a heterocyclic group, and Rxe2x80x2 is an alkylene group having 2 to 21 carbon atoms or a phenylenedialkylene group having 8 to 12 carbon atoms.
Examples of the acid residue include halide, sulfate, sulfite, bisulfite, thiosulfate, sulfide, polysulfide, hydrogen sulfide, thiocyanate, carbonate, bicarbonate, nitrate, carboxylate, borate, phosphate, biphosphate, phosphite, perchlorate, bifluoride, arsenate, ferricyanide, ferrocyanide, molybdate, selenate, selenite, uranate, tungstate, etc.
Specific examples of the quaternary ammonium salt include quaternary alkyl- and aralkyl-ammonium salts (e.g. trimethylbenzylammonium chloride, triethylbenzylammonium chloride, dimethyldecylbenzylammonium chloride, triethylbenzylammonium chloride, myristylbenzyldimethylammonium chloride, dodecyltrimethylammonium chloride, dimethyltetradecylbenzylammonium chlroride, trimethyltetradecylammonium chloride, coconuttrimethylammonium chloride, stearyltrimethylammonium chloride, distearyldimethylammonium chloride, tetrabutylammonium hydroxide, 1,4-phenylenedimethylenebistrimethylammonium dichloride, 1,4-phenylenedimethylenebistriethylammonium dichloride, ethylenebistriethylammonium dibromide, etc.), quaternary 1,8-diaza-bicyclo[5.4.0]-7-undecenium salts (e.g. 8-methyl-1,8-diaza-bicyclo[5.4.0]-7-undecenium chloride, 8-methyl-1,8-diaza-bicyclo[5.4.0]-7-undecenium iodide, 8-methyl-1,8-diaza-bicyclo[5.4.0]-7-undecenium hydroxide, 8-methyl-1,8-diaza-bicyclo[5.4.0]-7-undecenium methylsulfate, 8-methyl-1,8-diaza-bicyclo[5.4.0]-7-undecenium bromide, 8-propyl-1,8-diaza-bicyclo[5.4.0]-7-undecenium bromide, 8-dodecyl-1,8-diaza-bicyclo[5.4.0]-7-undecenium chloride, 8-dodecyl-1,8-diaza-bicyclo[5.4.0]-7-undecenium hydroxide, 8-eicosyl-1,8-diaza-bicyclo[5.4.0]-7-undecenium chloride, 8-tetracosyl-1,8-diaza-bicyclo[5.4.0]-7-undecenium chloride, 8-benzyl-1,8-diaza-bicyclo[5.4.0]-7-undecenium chloride, 8-benzyl-1,8-diaza-bicyclo[5.4.0]-7-undecenium hydroxide, 8-phenetyl-1,8-diaza-bicyclo[5.4.0]-7-undecenium chloride, 8-(3-phenylpropyl)-1,8-diaza-bicyclo[5.4.0]-7-undecenium chloride, etc.), and the like.
Furthermore, as curing accelerators, salts of the following tertiary amines with inorganic or organic acids can be used:
Tertiary Amine
A tertiary amine of the formula:
NR3 or R2Nxe2x80x94Rxe2x80x2xe2x80x94NR2
wherein R groups are the same or different and each is an alkyl or alkenyl group having 1 to 20 carbon atoms, or an aryl group having 6 to 20 carbon atoms, or two or more R groups together form a carbon ring or a heterocyclic group, and Rxe2x80x2 is an alkylene group having 2 to 21 carbon atoms or a phenylenedialkylene group having 8 to 12 carbon atoms.
Specific examples of the tertiary amines include trimethylamine, triethylamine, tri-n-propylamine, tri-n-butylamine, triisobutylamine, methyldiethylamine, dimethylethylamine, dimethyl-n-propylamine, dimethyl-n-butylamine, dimethylisobutylamine, dimethylisopropylamine, dimethyl-sec.-butylamine, dimethyl-tert.-butylamine, triallylamine, diallylmethylamine, allyldimethylamine, benzyldimethylamine, benzyldiethylemine, N-allylpiperidine, N-ethylpiperidine, N-butylpiperizine, N-methylpyrrolidine, N-cyclohexylpyrrolidine, N-n-butylpyrrolidine, N-ethylpyrrolidine, N-benzylpyrrolidine, 2,4,6-trimethylpyridine, etc.
Examples of the inorganic or organic acids which form the salts include HCl, HBr, HF, (C2H5)3NH+Clxe2x88x92, (C2H5)3NH+NO3xe2x88x92, 2(C2H5)3NH+SO42xe2x88x92, 2(C2H5)3NH+CO32xe2x88x92, (C2H5)3NH+ROxe2x88x92, (C2H5)3NH+RCOOxe2x88x92, (C4H9)3NH+Clxe2x88x92, (C4H9)3NH+NO3xe2x88x92, 2(C4H9)3NH+SO42xe2x88x92, 2(C4H9)3NH+CO32xe2x88x92, (C4H9)3NH+ROxe2x88x92, (C4H9)3NH+RCOOxe2x88x92 wherein R is an alkyl or alkenyl group having 1 to 20 atoms or an aryl group having 6 to 20 carbon atoms.
Primary or secondary amines are not preferable since they induce reactions other than the polyol curing. Amines having pKa of less than 8 are not preferable, since they achieve the low curing rate and thus the coating film has low strength.
In the present invention, an organic acid may be added to the composition to improve the shelf stability of the composition. As the organic acid, one having 1 to 12 carbon atoms, preferably 1 to 4 carbon atoms is used. Among them, those having 9 or more carbon atoms are less preferable since they remain in the coating film formed from the composition. Preferred organic acids are monocarboxylic acids such as formic acid, acetic acid, propionic acid, etc. and dicarboxylic acids such as oxalic acid, malonic acid, succinic acid, etc.
The organic acids suppress the polyol curing of the fluorine-containing copolymer contained in the composition during the storage. When the composition is coated, dried and baked, the organic acid is evaporated or thermally decomposed, the basic compound accelerates the curing reaction. Thus, the organic acid is categorized into xe2x80x9ccuring acceleratorsxe2x80x9d in the present invention.
The compounded amounts of the components in the composition according to the present invention are as follows:
The basic polyol curing agent is used in an amount of 0.1 to 10 wt. parts, preferably 0.5 to 5 wt. parts, and the curing accelerator is used in an amount of 0 to 10 wt. parts, preferably 0.01 to 5 wt. parts, all based on 100 wt. parts of the fluorine-containing copolymer.
When the amount of the curing accelerator is less than the above lower limit, it may be difficult to cure the copolymer. When the amount of the curing accelerator exceeds the above upper limit, it may be difficult to control the curing.
In addition to the polyol curing agent and the curing accelerator, the composition of the present invention may contain various additives which are usually added to the fluoroelastomer compositions, for example, fillers, colorants, acid-acceptors, etc.
Examples of the filler include carbon black, white carbon, calcium carbonate, barium sulfate, etc., and examples of the colorant include inorganic pigments, mixed oxide pigments, etc.
Examples of the acid-acceptor include magnesium oxide, lead oxide, zinc oxide, lead carbonate, zinc carbonate, and complex salts such as hydrotalcite. Acid-acceptors having a high activity such as calcium hydroxide are less preferable since the composition tends to gel. Acid-acceptors having lower pKa than that of the above basic compounds are preferable. When the acid-acceptors have high pKa, the composition tends to gel. In general, the acid-acceptor is used in an amount of 1 to 40 wt. parts per 100 wt. parts of the fluorine-containing copolymer in accordance with the activity of the acid-acceptor.
Furthermore, the composition of the present invention may contain 5 to 900 wt. parts of a fluororesin or a terminal-modified perfluoropolyether (a perfluoropolyether compound having a terminal functional group reactive with the fluorine-containing copolymer such as xe2x80x94NH2, xe2x80x94CH2OH, etc.) per 100 wt. parts of the fluorine-containing copolymer. Thereby, the non-stick property can be imparted to the coating film formed from the composition.
Examples of the fluororesin include polyvinylidene fluoride (PVdF), ethylene-tetrafluoroethylene copolymers (ETFE), polychlorotrifluoroethylene (CTFE), tetrafluoroethylene-hexafluoropropylene-perfluoroalkyl vinyl ether copolymer (EPA), tetrafluoroethylene-hexafluoropropylene copolymers (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether (PFA), polytetrafluoroethylene (PTFE), etc. Among them, tetrafluoroethylene base copolymers are preferable in view of the non-stick property. The fluororesins are preferably used in the form of an aqueous dispersion in view of the dispersibility.
The composition of the present invention can be applied and cured in the same manners as those used to apply and cure the conventional curable compositions of the fluorine-containing copolymers. That is, according to the properties of the compositions, they are applied to an article to be coated by brush coating, spray coating, dip coating, flow coating, dispenser coating, screen coating, etc., and sufficiently dried, followed by baking at a temperature of 150 to 300xc2x0 C. for 10 to 120 minutes.
The surface of the article to be coated is preferably defatted or washed. It is also preferable to form a primer layer on the surface of the article to be coated to improve the adhesion of the composition to the article. Examples of the primer include silane primers, silicone primers, etc.
A surface layer may be formed on the coating film formed from the composition of the present invention. The surface layer can be formed from the above fluororesins and/or the terminal-modified perfluoropolyethers by conventional methods.
The curable composition of the fluorine-containing elastic copolymer according to the present invention is characterized in that it has better shelf stability than an aqueous composition curable with a polyamine curing agent, although the composition of the present invention is a one-fluid type aqueous dispersion. Furthermore, the composition of the present invention can contain the solid of the fluorine-containing copolymer at a higher concentration than the solvent-based polyol-curing composition.
Examples of articles or substrates to be coated with the composition of the present invention are as follows:
metals (e.g. iron, stainless steel, copper, aluminum, brass, etc.), glass products (e.g. glass plates, woven and non-woven fabric of glass fiber, etc.), molded articles or coated articles of general resins or heat-resistant resins (e.g. polypropylene, polyoxymethylene, polyimide, polyamideimide, polysulfone, polyethersulfone, polyether ether ketone, etc.), molded articles or coated articles of general rubbers (e.g. styrene-butadiene rubber (SBR), isobutylene-isoprene rubber, nitrile-butadiene rubber (NBR), etyrene-propylene rubber (EPDM), etc.) and heat-resistant rubbers (e.g. silicone rubber, fluoroelastomer, etc.), woven and non-woven fabric of natural and synthetic fibers, and the like.
The coating layers formed from the composition of the present invention can be used in various fields which require heat resistance, solvent resistance, lubrication and/or non-stick properties. Specific examples of the applications include rolls (e.g. fixing rolls, press rolls, etc.) and conveying belts for OA equipment such as copying machines, printers, facsimiles, etc.; sheets and belts; O-rings, diaphragms, chemical-resistant tubes, fuel hoses, valve seals, gaskets for chemical plants, engine gaskets, and the like.